The present invention relates to a draft limiting device for a flue gas passage of a fireplace. The device includes a flap arranged to swivel about the axis of a swing shaft, and the device is located in a secondary air inlet of the flue gas passage. In response to a differential pressure which acts on the flap, the flap moves against a restoring moment from a closed position to a fully open position to admit air from a room to the flue gas passage.
Draft limiting devices are known which adjust the draft in the flue gas passage to an optimum value for a combustion process, taking into consideration that too great a draft in the flue gas passage leads to excessive combustion air. Since fresh air enters through a secondary air inlet, the draft in the flue gas passage can be reduced and thus an optimum amount of combustion air can be fed to the combustion process.
Since the draft in the flue gas passage depends on operating variables, e.g., the ambient temperature or the chimney temperature, the fresh air supply through the secondary air inlet cannot be set permanently. Rather, some control is necessary in order to keep the draft nearly constant.
Test devices are known which attempt to keep the draft in the flue gas passage nearly constant, independent of the above-mentioned variables. However, these draft limiting devices were insufficient because the desired value set for the draft, i.e., the reduced pressure which prevails at the fireplace, did not remain substantially constant, especially when the chimney draft was greater.
It was discovered that the unsatisfactory performance of the known draft limiting devices is due to the fact that the restoring moment on the flap becomes increasingly larger as the flap approaches an open position. However, when the flap is in the open position, the force which acts on the flap and which tends to deflect the flap toward the open position is at a minimum. This is true for both of the force components which originate from the static pressure differential conditions, as well as from the flow conditions. Accordingly, the known draft limiting devices open only partially because, as they move from the closed to the open position, the restoring force on the flap becomes greater while the force urging the flap toward the open position becomes increasingly smaller. The draft in the flue gas passage at the fireplace therefore does not remain constant, but increses in an undesirable manner with increasing chimney draft.
An object of the invention is to construct a draft limiting device such that an at least substantially constant draft and, consequently, a reduced pressure in the region of the fireplace can be maintained over a large range of variable operating parameters.
According to the invention, restoring means on the flap of a draft limiting device provides a restoring moment to the flap which either remains substantially constant or becomes smaller over at least a portion of the path of movement of the flap as it moves between a closed and a fully open position.
In one embodiment of a draft limiting device according to the invention, the restoring moment does not increase as the flap moves from the closed toward the open position, but remains constant or becomes smaller, so that the static and dynamic force components acting on the flap serve to move the flap further toward the open position than has been possible up to now.
It has been demonstrated that a constant effective draft at the fireplace can be achieved when the restoring moment is substantially constant for a first part of the path of movement of the flap which first part includes the closed position, and when the restoring moment becomes smaller in a second part of the path of movement which second part includes the open position.
The arrangement of the invention is not only advantageous because of the constant draft at the fireplace, but also because the restoring moment is at its highest value in the closed position of the flap. The high restoring moment permits the flap reliably to remain closed up to a desired value of the effective draft at the fireplace. Therefore, when the draft is low, i.e., below the desired value, the flap does not start to swing as in the known draft limiting devices. While in the known devices, the flap always swings open even when the chimney draft is below the value set for the draft at the fireplace, the device of the present invention is arranged to prevent the flap from swinging open under such conditions. Only when the value of the chimney draft corresponds substantially to the value set at the flap does the flap begin to swing open.
According to a preferred embodiment of the invention, the restoring moment which is effective in the closed position and the restoring moment which is effective in the fully open position can be adjusted to a great extent independently of one another. This feature has the advantage that a pressure threshold at which the flap starts to open can be set when the flap is in the closed position, and that a restoring moment still exists in the open position which, depending on the desired control function, can be set to a value which is as small as possible. This makes it possible to compensate for tolerances such as are required when the restoring moment is produced by gravitational forces and, therefore, depends on the position of the device when installed, as well as dimensional variations from production of the device.
The draft limiting device of the invention can also be constructed with return springs. Return springs with a substantially flat force characteristic are available in the form of, e.g., gas springs. Spring arrangements with a decreasing force characteristic can be realized with gear means, e.g., by the selection of the system points of the spring arrangement.
When arranging the flap as a pendulum which is supported by a horizontal swing shaft for swiveling movement between the closed and the open position, a restoring torque or moment developed by gravitational force can be used. The concept of the invention then can be easily realized by providing that the center of gravity of the flap arrangement which includes all parts fixed to the flap, lies substantially on the same level as the axis of the swing shaft in the closed position. Then, when the flap approaches the open position, the center of gravity follows a circular path, and the horizontal distance or moment arm from the center of gravity to the swing shaft axis decreases in a co-sinusoidal relation. This results in a decreasing value of the restoring moment as the flap moves toward the open position.
In the above embodiment, the value of the restoring moment which is provided in the closed position can be set in a simple manner by making the center of gravity of the flap arrangement adjustable in the horizontal direction. The restoring moment which is provided in the open position can be set by making the center of gravity of the flap arrangement adjustable in the vertical direction when the flap arrangement is viewed in the closed position.
The above two settings for the center of gravity can be implemented in a simple manner by providing an elongated arm extending transversely of and horizontally from the swing shaft (when the flap arrangement is viewed in the closed position), and a weight on the arm which can be set at a desired position along the longitudinal direction of the arm. Further, the arm is secured to the swing shaft so that it can be adjustably rotated about the axis of the swing shaft and remain fixed at a desired position relative to the swing shaft. Although the longitudinal axis of the arm need not intersect the axis of the swing shaft, it is preferable that both axes intersect one another for reasons of structural simplicity. It is also preferable that the arm face toward the room, thereby facilitating its operation and allowing the arm weight to be easily accessible.
The flap preferably is constructed so that during the change from the closed to the open position, a secondary air passage is defined between only one side of the flap and the secondary air inlet. This is contrary to the known draft limiting devices in which a secondary air passage is formed on both sides of the flap when the flap is opened. The disadvantage of the known arrangement is that an airfoil condition results from the flow of the secondary air, which adds to the restoring force as the flap opening increases. This situation does not occur with the present device which acts to maintain a constant draft at the fireplace.
The formation of a single air passage on only one side of the flap is achieved by placing the swing shaft in the region of an inside edge of the secondary air inlet. While such an arrangement might appear to make it difficult to locate the center of gravity of the closed flap arrangement on the same level as the swing shaft axis, a preferred embodiment of the present device overcomes such a problem by providing a raised shield member fixed to the flap. Therefore, when the swing shaft is arranged parallel to an inside edge of the secondary air inlet, the entry of the secondary air inlet located between the swing shaft and the inside edge always remains closed. The shield member may be in the form of, e.g., a semi-cylinder having a center of radius which coincides with the axis of the swing shaft, and having a radius which corresponds to the distance between the swing shaft axis and the adjacent inside edge of the secondary air inlet. The semi-cylinder may be arranged so that its flat outer surface which is parallel to the center of radius abuts the surface of the flap. By suitably locating the semi-cylinder, the center of gravity of the entire flap arrangement can be brought to the desired location. The shield member can be arranged, for example, so that it extends at least partly over the smaller portion of the flap which is bounded by the swing shaft, so that the shield member acts as a counterbalance weight for the remaining larger portion of the flap at the desired setting of the center of gravity.
In order to provide for fine adjustment of the center of gravity, it is preferred that the arm weight be substantially lighter than the counterbalance weight of the shield member. Accordingly, fine adjustment of the center of gravity is achieved easily by moving the arm weight over relatively large distances along the arm.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.